There are circumstances, fortunately rare, in which an aircraft mounted gas turbine engine ceases functioning during flight. If the engine is undamaged, an attempt is usually made to restart it. Normally this is not difficult to achieve since the engine continues to rotate as a result of the ram air flow which passes through it. However if the engine was operating at full power or near full power immediately prior to its ceasing to function, it can be difficult to restart. This is due to residual heat within certain parts of the engine heating up the air flowing through the engine to undesirably high levels. Such heated air has a particularly prejudicial effect upon the operating characteristics of the high pressure compressor of the engine. Specifically it has been found that the operating characteristics are degraded to such an extent that it is incapable of providing an air pressure rise sufficient to support the restarting and continued operation of the engine.